In a cold plastic process of metal materials such as a wire drawing process, a drawing process, a squeezing process and the like, a film for lubricating a processed material and a tool in order to prevent them from adhering to each other is generally formed on the surface of a processed metal material. Regarding a method for forming a film on the surface of a processed metal material, it is broadly classified into a method (I) which includes fixing a protective film material or a lubricant directly to the surface of the processed metal material, and a method (II) which includes forming a protective film on the surface of the processed metal material by applying a chemical conversion treatment and then forming a lubricative film on the upper layer of the protective film.
Since it is easy to apply the method (I) which includes adhering a protective film material or a lubricant directly to the surface of the processed metal material, it is advantageous in that there is substantially no need to manage a treatment liquid. However, a film formed according to the method (I) does not have sufficient adhesion to an underlying metal. In particular, in a haze process in which the surface of a processed metal material is in contact with a mold under strong surface pressure, the lubricative film may be detached from the metal material during hazing. In addition, in a process in which a processed metal material is greatly deformed, the lubricative film may not comply with the deformation of the processed metal material so that the lubricative film may be exfoliated and detached therefrom. In addition to a defect in the lubricative film, an extremely thin film portion and a film breakage portion may be readily generated even on a protective film formed on an underlying metal. Therefore, the film formed according to the method (I) may not sufficiently exhibit its function as a protective film. For that reason, in a cold plastic process such as a metal wire drawing process, a drawing process, and the like, the method (II) is frequently applied which includes forming a protective film by applying a chemical conversion treatment and then forming a lubricative film on the upper layer of the protective film in order to obtain a film having high lubricity and strong adhesion.
A phosphate compound-based film and an oxalate compound film are known as the protective films formed by a chemical conversion treatment. These protective films have high adhesion to the surface of a processed metal material. However, the protective films have insufficient corrosion resistance. As a result, corrosion and rust are generated on a metal material due to environmental changes occurring when transporting the metal material, thereby considerably deteriorating a product value. Therefore, the protective films are not preferable to use from a viewpoint of quality. For that reason, it is strongly required to have a film having both excellent corrosion resistance and lubricity sustainable in severe processes, and a film for solving the above-mentioned problems and a method for forming the film have been proposed.
When a crystalline inorganic compound film is used, a base metal may be exposed to the outside through a gap between the crystals. Therefore, the exposed portion of the metal is readily rusted and the corrosion resistance deteriorates. In Patent Document 1, a technique is proposed in which a mixed film formed by mixing a crystalline inorganic compound, such as sulfate, and an amorphous inorganic material, such as silicate, is used as a protective film as a support, and improvement in function as a protective film is made by applying the technique. However, the amorphous inorganic material film, which is the outer surface, has poor adhesion to a lubricative film. Therefore, in Patent Document 1, a method is proposed which includes setting the surface roughness in the amorphous inorganic material film, which is the outer surface, to 2 to 10 μm to increase retentive property and adhesion of a lubricative film. However, it is impossible to obtain the adhesion and lubricity required for severe processes such as a wire drawing process, a drawing process, a squeezing process and the like.
In Patent Document 2, a protective film is disclosed which is obtained according to a simple method which includes mixing specific alkaline silicate with water. Alkaline silicate exhibits satisfactory corrosion resistance as a protective film. However, when a lubricative component is applied onto the protective film, adhesion of the protective film to the lubricative film is insufficient, thereby deteriorating lubricity. In addition, when a lubricative component is contained in the film, it is impossible for the film to have both lubricity and protective property (corrosion resistance) and both properties deteriorate.
In Patent Document 3, as a method for producing a satisfactory film with anti-rust and anti-corrosion properties, the present inventors have proposed a method for producing an oxide film or a hydroxide film of a metal, which includes applying a liquid-phase precipitation method in which a fluorine compound aqueous solution is applied on the surface of an underlying metal. In Patent Document 4, the present inventors have proposed forming a film made of an oxyacid compound or an oxyacid hydrogen compound, which is an element in an IVA group, on the surface of an underlying metal as a satisfactory film for anti-rust and anti-corrosion.
According to the method described in Patent Document 3, it is possible to obtain a zirconium oxide film which exhibits corrosion resistance because of the barrier property. However, when a metal material is subjected to severe processes such as a wire drawing process, a drawing process, a squeezing process and the like, the film may not sufficiently exhibit excellent corrosion resistance. On the other hand, according to the method described in Patent Document 4, it is possible to obtain both process compliance of an oxyacid compound or an oxyacid hydrogen compound, which is an element in an IVA group, such as zirconium, and corrosion resistance due to the barrier property. However, since a structure of the film is simple and is not a laminated structure having an inclined structure, the film may not exhibit sufficient corrosion resistance under corrosive conditions containing a complicated corrosion factor and a strong corrosion factor.